To evaluate the working memory (WM) performance of adults with attention-deficit/hyperactivity disorder (ADHD) using event-related potential (ERP), and to examine whether there is output dysfunction of their WM resource under increasing memory load.

Forty-three adult ADHD patients, and 40 healthy controls matched in gender ratio, age and IQ were recruited to participate in the study. All participants were required to perform a verbal n-back test with EEG recording. Behavioral indices including reaction time (RT), intra-individual reaction time variability (ISD) and accuracy rate were recorded by software E-prime. Meanwhile, electrophysiology indexes were recorded by EEG/ERP recording system EGI. Comparisons were conducted between ADHD patients and normal controls on their behavioral performances of RT, accuracy rate and ISD and their electrophysiology component of P300. Pearson correlation analysis was used to compare the correlation between behavioral, electrophysiological indicators and clinical symptoms.

Behaviorally, ADHD patients performed with a longer RT than healthy controls under 1-back ((505±100) ms vs. (457±75) ms, F_{(1, 81)}=5.94, P=0.017) and 2-back ((573±125) ms vs. (511±100) ms, F_{(1, 81)}=6.05, P=0.016). Main effect of group on ISD was found (F_{(1, 81)}=6.03, P=0.016), where adult ADHD patients performed significantly higher than healthy controls. Neurophysiolocally, significantly lower amplitude of P300 component under 2-back task was found among ADHD patients compared to healthy controls (F_{(1, 78)}=4.14, P=0.045). Correlation analysis: the more severe the hyperactivity symptoms of adult ADHD, the longer reaction time to complete the 2-back task (r=0.303, P=0.48). The more severe the hyperactivity symptoms of adult ADHD, the lower P300 amplitude to complete the n-back task. The more severe the attention deficit symptom of adult ADHD, the longer P300 latency to complete the n-back task.

Adult ADHD patients may have prolonged response time, increased variability and decreased P300 amplitude compared to the control subjects in n-back task, which is correlated to clinical symptoms. These signs suggest the impairment in retrieval of working memory, and an unstable attention, as well as slower information processing speed among adult patients with ADHD.

Attention deficit disorder with hyperactivity; Cross-sectional study; Event-related potentials, P300; Reaction time; Working memory